The heart of the bivalve mollusc, Modiolus demissus, is structurally and functionally an analog of the vertebrate myocardium, and is thus a favorable model system for studying the general mechanisms underlying rhythmical activity in cardiac muscle. This investigation will focus on two agents which profoundly modify rhythmicity: 5-hydroxytryptamine (5HT) and FMRFamide. 5HT is the molluscan cardioexcitatory neurotransmitter; its action parallels almost exactly that of adrenaline on the vertebrate heart. FMRFamide, a ubiquitous molluscan neuropeptide: Phe-Met-Arg-Phe-NH2, is a cardioexcitatory and antiarrhythmic agent, inducing spontaneous rhythmical activity in quiescent molluscan hearts and augmenting the beat of active ones. The study of mechanism will focus on the distribution of calcium ion and its movement during excitation-contraction coupling. The effects of 5HT and FMRFamide on 45Ca ions efflux, and the specificity of those effects, will be examined in detail. Changes in efflux produced by the two agonists will be analyzed with the aid of agents which are known to affect intracellular Ca ions movements; e.g., La ions, divalent ions; ouabain, Verapamil and D-600. An attempt will also be made to reproduce some of the whole heart responses on isolated sarcolemmal vesicles; in these experiments, net calcium fluxes will be monitored with a specific ion electrode. The possibility that FMRFamide is a neurosecretory product will be tested by subcellular fractionation of Macrocallista ganglia. A radioimmunoassay (RIA) for FMRFamide will be developed to facilitate the identification of the fraction containing the tetrapeptide. The RIA will be verified by bioassay, and the morphology of the FMRFamide-containing cell fractions will be determined by electron microscopy.